Figure 3.12 Dry partitionNew buildings have internal partitions constructed of timber studding with light plasterboard facing, as illustrated in Figure 3.12, or preconstructed partitioning. This is particularly the case with proprietary industrialized systems of building. Such a partition has a void within it which is used for engineering services, and this is also a situation in which PVC sheathed cable without further protection is the most suitable system of wiring, provided that the cable is at least 50mm from the surface. Voids of this sort, in which PVC sheathed cable is run, are usually sufficiently accessible to make rewiring, if not easy, at least possible. Fixing of accessories is made easy by the use of dry liner boxes as illustrated in Chapter 1 (Figure 1.1). It may happen that the building structure is such that PVC sheathed cable on its own is the most suitable system to use, but that there are a few places where cable has to drop in plastered walls or run across floors. It is then desirable to give the cable additional protection at these places by running it inside conduit at these places only. This has the additional advantage of making rewiring easier. As the conduit is used only for short lengths for local protection, both light-gauge steel and PVC conduit are suitable. PVC sheathed cable may be buried in plaster without damage. There is, however, the possibility that nails may be accidentally driven into the cables when pictures are being fixed to walls. Ideally, the cable should therefore be protected by conduit, but if there is not sufficient depth of plaster to make this possible it can still be given protection by shallow rigid PVC or galvanized metal channelling as shown in Figure 3.13. Many authorities feel that even this is not necessary. BS 7671 specifies areas within a wall in which cables are installed at a depth of less than 50mm, where the cables do not need additional protection. Guidance is also given in the IEE Guidance Note

New buildings have internal partitions constructed of timber studding with light
plasterboard facing, as illustrated in Figure 3.12, or preconstructed partitioning. This is
particularly the case with proprietary industrialized systems of building. Such a partition
has a void within it which is used for engineering services, and this is also a situation in
which PVC sheathed cable without further protection is the most suitable system of
wiring, provided that the cable is at least 50mm from the surface. Voids of this sort, in
which PVC sheathed cable is run, are usually sufficiently accessible to make rewiring, if
not easy, at least possible. Fixing of accessories is made easy by the use of dry liner
boxes as illustrated in Chapter 1 (Figure 1.1).
It may happen that the building structure is such that PVC sheathed cable on its own is
the most suitable system to use, but that there are a few places where cable has to drop in
plastered walls or run across floors. It is then desirable to give the cable additional
protection at these places by running it inside conduit at these places only. This has the
additional advantage of making rewiring easier. As the conduit is used only for short
lengths for local protection, both light-gauge steel and PVC conduit are suitable.
PVC sheathed cable may be buried in plaster without damage. There is, however, the
possibility that nails may be accidentally driven into the cables when pictures are being
fixed to walls. Ideally, the cable should therefore be protected by conduit, but if there is
not sufficient depth of plaster to make this possible it can still be given protection by
shallow rigid PVC or galvanized metal channelling as shown in Figure 3.13. Many
authorities feel that even this is not necessary. BS 7671 specifies areas within a wall in
which cables are installed at a depth of less than 50mm, where the cables do not need
additional protection. Guidance is also given in the IEE Guidance Note
1 and the IEE On-Site Guide. The basic rule is that cables can be run under plaster at a
depth of less that 50mm without protection, in straight lines between accessories, also
within 150mm of a change of direction of the wall.
There are situations where the appearance of an installation is of secondary
importance, and where at the same time a surface system will not receive rough usage.
Such a case might occur in an old building used for commercial purposes or in simple
huts at a holiday camp. PVC sheathed cable may then be run on exposed surfaces without
further protection. Since it is visible it will not be damaged accidentally by people trying
to fix things to the walls.
PVC sheathed cable is fixed with moulded plastic clips. An example is illustrated in
Figure 3.14. The clips should be spaced appropriate to the size of cables. IEE Guidance
Note 1 Selection and Erection of Equipment and the IEE On-Site Guide give guidance on
the spacing of cable clips.